Presynaptic Cytomatrix Proteins

The Cytomatrix Assembled at the active Zone (CAZ) of a presynaptic terminal displays electron-dense appearance and defines the center of the synaptic vesicle release. The protein constituents of CAZ are multiple-domain scaffolds that interact extensively with each other and also with an ensemble of synaptic vesicle proteins to ensure docking, fusion, and recycling. Reflecting the central roles of the active zone in synaptic transmission, CAZ proteins are highly conserved throughout evolution. As the nervous system increases complexity and diversity in types of neurons and synapses, CAZ proteins expand in the number of gene and protein isoforms and interacting partners. This chapter summarizes the discovery of the core CAZ proteins and current knowledge of their functions.

Nanoscopical Analysis Reveals an Orderly Arrangement of the Presynaptic Scaffold Protein Bassoon at the Golgi-Apparatus

Bassoon is a core scaffold protein of the presynaptic active zone. In brain synapses, the C-terminus of Bassoon is oriented toward the plasma membrane and its N-terminus is oriented toward synaptic vesicles. At the Golgi-apparatus, Bassoon is thought to assemble active zone precursor structures, but whether it is arranged in an orderly fashion is unknown. Understanding the topology of this large scaffold protein is important for models of active zone biogenesis. Using stimulated emission depletion nanoscopy in cultured hippocampal neurons, we found that an N-terminal intramolecular tag of recombinant Bassoon, but not C-terminal tag, colocalized with markers of the trans-Golgi network (TGN). The N-terminus of Bassoon was located between 48 and 69 nm away from TGN38, while its C-terminus was located between 100 and 115 nm away from TGN38. Sequences within the first 95 amino acids of Bassoon were required for this arrangement. Our results indicate that, at the Golgi-apparatus, Bassoon is oriented with its N-terminus toward and its C-terminus away from the trans Golgi network membrane. Moreover, they suggest that Bassoon is an extended molecule at the trans Golgi network with the distance between amino acids 97 and 3,938, estimated to be between 46 and 52 nm. Our data are consistent with a model, in which the N-terminus of Bassoon binds to the membranes of the trans-Golgi network, while the C-terminus associates with active zone components, thus reflecting the topographic arrangement characteristic of synapses also at the Golgi-apparatus.

CtBP1-Mediated Membrane Fission Contributes to Effective Recycling of Synaptic Vesicles

Compensatory endocytosis of released synaptic vesicles (SVs) relies on coordinated signaling at the lipid-protein interface. Here, we address the synaptic function of C-terminal binding protein 1 (CtBP1), a ubiquitous regulator of gene expression and membrane trafficking in cultured hippocampal neurons. In the absence of CtBP1, synapses form in greater density and show changes in SV distribution and size. The increased basal neurotransmission and enhanced synaptic depression could be attributed to a higher vesicular release probability and a smaller fraction of release-competent SVs, respectively. Rescue experiments with specifically targeted constructs indicate that, while synaptogenesis and release probability are controlled by nuclear CtBP1, the efficient recycling of SVs relies on its synaptic expression. The ability of presynaptic CtBP1 to facilitate compensatory endocytosis depends on its membrane-fission activity and the activation of the lipid-metabolizing enzyme PLD1. Thus, CtBP1 regulates SV recycling by promoting a permissive lipid environment for compensatory endocytosis.

The absence of functional bassoon at cone photoreceptor ribbon synapses affects signal transmission at Off cone bipolar cell contacts in mouse retina

AIM

Off cone bipolar cells of the mammalian retina connect to cone photoreceptor synaptic terminals via non-invaginating flat contacts at a considerable distance from the only established neurotransmitter release site so far, the synaptic ribbon. Diffusion from the ribbon synaptic active zone is considered the most likely mechanism for the neurotransmitter glutamate to reach postsynaptic receptors on the dendritic tips of Off cone bipolar cells. We used a mutant mouse with functionally impaired photoreceptor ribbon synapses to investigate the importance of intact ribbon synaptic active zones for signal transmission at Off cone bipolar cell contacts.

METHODS

Whole-cell patch-clamp recordings from Off cone bipolar cells in a horizontal slice preparation of wildtype (Bsn^wt ) and mutant (Bsn^ΔEx4/5 ) mouse retina were applied to investigate signal transmission between cone photoreceptors and Off cone bipolar cells. The distribution of postsynaptic glutamate receptors in Off cone bipolar cell dendrites was studied using multiplex immunocytochemistry.

RESULTS

Tonic synaptic activity and evoked release were significantly reduced in mutant animals. Vesicle replenishment rates and the size of the readily releasable pool were likewise decreased. The precisely timed transient current response to light offset changed to a sustained response in the mutant, exemplified by random release events only loosely time-locked to the stimulus. The kainate receptor distribution in postsynaptic Off cone bipolar cell dendritic contacts in Bsn^ΔEx4/5 mice was largely disturbed.

CONCLUSION

Our results suggest a major role of functional ribbon synaptic active zones for signal transmission and postsynaptic glutamate receptor organization at flat Off cone bipolar cell contacts.

Effects of exercise training on neuromuscular junctions and their active zones in young and aged muscles

The neuromuscular junction (NMJ) connects the motor neuron with myofibers allowing muscle contraction. Both aging and increased activity result in NMJ remodeling. Here, the effects of exercise were examined in young and aged soleus muscles. Using immunofluorescent staining procedures, cellular and active zone components of the NMJ were quantified following a treadmill running program. Immunofluorescence was employed to determine myofiber profiles (size and type). Two-way analysis of variance procedures with main effects of age and treatment showed that when analyzing NMJs at the cellular level, significant (p ≤ 0.05) effects were identified for age, but not treatment. However, when examining subcellular active zones, effects for exercise, but not for age, were detected. Myofiber cross-sectional area showed that aging elicited atrophy and that among younger muscles endurance exercise training yielded decrements in myofiber size. Conversely, among aged muscles training elicited whole muscle and myofiber trends (p < 0.10) toward hypertrophy. Thus, different components of the neuromuscular system harbor unique sensitivities to various stimuli enabling proper adaptations to attain optimal function under differing conditions.

Herpesviruses, autoimmunity and epilepsy: Peptide sharing and potential cross-reactivity with human synaptic proteins

Aggregation of immuno-proteomic data reveals that i) herpesviruses and synaptic proteins -in particular Synapsin-1 and Bassoon - share a large number of hexapeptides that also recur in hundreds of epitopes experimentally validated as immunopositive in the human host, and ii) the shared peptides are also spread among human epilepsy-related proteins. The data indicate that cross-reactive processes may be associated with pathogenetic mechanisms in epilepsy, thus suggesting a role of autoimmunity in etiopathology of epilepsies after herpesvirus-infections.

Signal transmission at invaginating cone photoreceptor synaptic contacts following deletion of the presynaptic cytomatrix protein Bassoon in mouse retina

AIM

A key feature of the mammalian retina is the segregation of visual information in parallel pathways, starting at the photoreceptor terminals. Cone photoreceptors establish synaptic contacts with On bipolar and horizontal cells at invaginating, ribbon-containing synaptic sites, whereas Off bipolar cells form flat, non-ribbon-containing contacts. The cytomatrix protein Bassoon anchors ribbons at the active zone, and its absence induces detachment of ribbons from the active zone. In this study we investigate the impact of a missing Bassoon on synaptic transmission at the first synapse of the visual system.

METHODS

Release properties of cone photoreceptors were studied in wild-type and mutant mouse retinae with a genetic disruption of the presynaptic cytomatrix protein Bassoon using whole-cell voltage-clamp recordings. Light and electron microscopy revealed the distribution of Ca²⁺ channels and synaptic vesicles, respectively, in both mouse lines.

RESULTS

Whole-cell recordings from postsynaptic horizontal cells of the two mouse lines showed that the presence of Bassoon (and a ribbon) enhanced the rate of exocytosis during tonic and evoked release by increasing synaptic vesicle pool size and replenishment rate, while at the same time slowing synaptic vesicle release. Furthermore, the number of Ca_v 1.4 channels and synaptic vesicles was significantly higher at wild-type than at Bassoon mutant synaptic sites.

CONCLUSION

The results of our study demonstrate that glutamate release from cone photoreceptor terminals can occur independent of a synaptic ribbon, but seems restricted to active zones, and they show the importance of a the synaptic ribbon in sustained and spatially and temporally synchronized neurotransmitter release.

Nutrient limitation affects presynaptic structures through dissociable Bassoon autophagic degradation and impaired vesicle release

Acute mismatch between metabolic requirements of neurons and nutrients/growth factors availability characterizes several neurological conditions such as traumatic brain injury, stroke and hypoglycemia. Although the effects of this mismatch have been investigated at cell biological level, the effects on synaptic structure and function are less clear. Since synaptic activity is the most energy-demanding neuronal function and it is directly linked to neuronal networks functionality, we have explored whether nutrient limitation (NL) affects the ultrastructure, function and composition of pre and postsynaptic terminals. We show that upon NL, presynaptic terminals show disorganized vesicle pools and reduced levels of the active zone protein Bassoon (but not of Piccolo). Moreover, NL triggers an impaired vesicle release, which is reversed by re-administration of glucose but not by the blockade of autophagic or proteasomal protein degradation. This reveals a dissociable correlation between presynaptic architecture and vesicle release, since restoring vesicle fusion does not necessarily depend from the rescue of Bassoon levels. Thus, our data show that the presynaptic compartment is highly sensitive to NL and the rescue of presynaptic function requires re-establishment of the metabolic supply rather than preventing local protein degradation.

Ablation of the presynaptic organizer Bassoon in excitatory neurons retards dentate gyrus maturation and enhances learning performance

Bassoon is a large scaffolding protein of the presynaptic active zone involved in the development of presynaptic terminals and in the regulation of neurotransmitter release at both excitatory and inhibitory brain synapses. Mice with constitutive ablation of the Bassoon (Bsn) gene display impaired presynaptic function, show sensory deficits and develop severe seizures. To specifically study the role of Bassoon at excitatory forebrain synapses and its relevance for control of behavior, we generated conditional knockout (Bsn cKO) mice by gene ablation through an Emx1 promoter-driven Cre recombinase. In these animals, we confirm selective loss of Bassoon from glutamatergic neurons of the forebrain. Behavioral assessment revealed that, in comparison to wild-type littermates, Bsn cKO mice display selectively enhanced contextual fear memory and increased novelty preference in a spatial discrimination/pattern separation task. These changes are accompanied by an augmentation of baseline synaptic transmission at medial perforant path to dentate gyrus (DG) synapses, as indicated by increased ratios of field excitatory postsynaptic potential slope to fiber volley amplitude. At the structural level, an increased complexity of apical dendrites of DG granule cells can be detected in Bsn cKO mice. In addition, alterations in the expression of cellular maturation markers and a lack of age-dependent decrease in excitability between juvenile and adult Bsn cKO mice are observed. Our data suggest that expression of Bassoon in excitatory forebrain neurons is required for the normal maturation of the DG and important for spatial and contextual memory.

Piccolo Promotes Vesicle Replenishment at a Fast Central Auditory Synapse

Piccolo and Bassoon are the two largest cytomatrix of the active zone (CAZ) proteins involved in scaffolding and regulating neurotransmitter release at presynaptic active zones (AZs), but have long been discussed as being functionally redundant. We employed genetic manipulation to bring forth and segregate the role of Piccolo from that of Bassoon at central auditory synapses of the cochlear nucleus—the endbulbs of Held. These synapses specialize in high frequency synaptic transmission, ideally poised to reveal even subtle deficits in the regulation of neurotransmitter release upon molecular perturbation. Combining semi-quantitative immunohistochemistry, electron microscopy, and in vitro and in vivo electrophysiology we first studied signal transmission in Piccolo-deficient mice. Our analysis was not confounded by a cochlear deficit, as a short isoform of Piccolo (“Piccolino”) present at the upstream ribbon synapses of cochlear inner hair cells (IHC), is unaffected by the mutation. Disruption of Piccolo increased the abundance of Bassoon at the AZs of endbulbs, while that of RIM1 was reduced and other CAZ proteins remained unaltered. Presynaptic fiber stimulation revealed smaller amplitude of the evoked excitatory postsynaptic currents (eEPSC), while eEPSC kinetics as well as miniature EPSCs (mEPSCs) remained unchanged. Cumulative analysis of eEPSC trains indicated that the reduced eEPSC amplitude of Piccolo-deficient endbulb synapses is primarily due to a reduced readily releasable pool (RRP) of synaptic vesicles (SV), as was corroborated by a reduction of vesicles at the AZ found on an ultrastructural level. Release probability seemed largely unaltered. Recovery from short-term depression was slowed. We then performed a physiological analysis of endbulb synapses from mice which, in addition to Piccolo deficiency, lacked one functional allele of the Bassoon gene. Analysis of the double-mutant endbulbs revealed an increase in release probability, while the synapses still exhibited the reduced RRP, and the impairment in SV replenishment was exacerbated. We propose additive roles of Piccolo and Bassoon in SV replenishment which in turn influences the organization and size of the RRP, and an additional role of Bassoon in regulation of release probability.

Neuronal LRP4 regulates synapse formation in the developing CNS

The low-density lipoprotein receptor-related protein 4 (LRP4) is essential in muscle fibers for the establishment of the neuromuscular junction. Here, we show that LRP4 is also expressed by embryonic cortical and hippocampal neurons, and that downregulation of LRP4 in these neurons causes a reduction in density of synapses and number of primary dendrites. Accordingly, overexpression of LRP4 in cultured neurons had the opposite effect inducing more but shorter primary dendrites with an increased number of spines. Transsynaptic tracing mediated by rabies virus revealed a reduced number of neurons presynaptic to the cortical neurons in which LRP4 was knocked down. Moreover, neuron-specific knockdown of LRP4 by in utero electroporation of LRP4 miRNA in vivo also resulted in neurons with fewer primary dendrites and a lower density of spines in the developing cortex and hippocampus. Collectively, our results demonstrate an essential and novel role of neuronal LRP4 in dendritic development and synaptogenesis in the CNS.

Vertebrate Presynaptic Active Zone Assembly: a Role Accomplished by Diverse Molecular and Cellular Mechanisms

Among all the biological systems in vertebrates, the central nervous system (CNS) is the most complex, and its function depends on specialized contacts among neurons called synapses. The assembly and organization of synapses must be exquisitely regulated for a normal brain function and network activity. There has been a tremendous effort in recent decades to understand the molecular and cellular mechanisms participating in the formation of new synapses and their organization, maintenance, and regulation. At the vertebrate presynapses, proteins such as Piccolo, Bassoon, RIM, RIM-BPs, CAST/ELKS, liprin-α, and Munc13 are constant residents and participate in multiple and dynamic interactions with other regulatory proteins, which define network activity and normal brain function. Here, we review the function of these active zone (AZ) proteins and diverse factors involved in AZ assembly and maintenance, with an emphasis on axonal trafficking of precursor vesicles, protein homo- and hetero-oligomeric interactions as a mechanism of AZ trapping and stabilization, and the role of F-actin in presynaptic assembly and its modulation by Wnt signaling.

The role of laminins in the organization and function of neuromuscular junctions

The synapse between motor neurons and skeletal muscle is known as the neuromuscular junction (NMJ). Proper alignment of presynaptic and post-synaptic structures of motor neurons and muscle fibers, respectively, is essential for efficient motor control of skeletal muscles. The synaptic cleft between these two cells is filled with basal lamina. Laminins are heterotrimer extracellular matrix molecules that are key members of the basal lamina. Laminin α4, α5, and β2 chains specifically localize to NMJs, and these laminin isoforms play a critical role in maintenance of NMJs and organization of synaptic vesicle release sites known as active zones. These individual laminin chains exert their role in organizing NMJs by binding to their receptors including integrins, dystroglycan, and voltage-gated calcium channels (VGCCs). Disruption of these laminins or the laminin-receptor interaction occurs in neuromuscular diseases including Pierson syndrome and Lambert-Eaton myasthenic syndrome (LEMS). Interventions to maintain proper level of laminins and their receptor interactions may be insightful in treating neuromuscular diseases and aging related degeneration of NMJs.

Role of Bassoon and Piccolo in Assembly and Molecular Organization of the Active Zone

Bassoon and Piccolo are two very large scaffolding proteins of the cytomatrix assembled at the active zone (CAZ) where neurotransmitter is released. They share regions of high sequence similarity distributed along their entire length and seem to share both overlapping and distinct functions in organizing the CAZ. Here, we survey our present knowledge on protein-protein interactions and recent progress in understanding of molecular functions of these two giant proteins. These include roles in the assembly of active zones (AZ), the localization of voltage-gated Ca²⁺ channels (VGCCs) in the vicinity of release sites, synaptic vesicle (SV) priming and in the case of Piccolo, a role in the dynamic assembly of the actin cytoskeleton. Piccolo and Bassoon are also important for the maintenance of presynaptic structure and function, as well as for the assembly of CAZ specializations such as synaptic ribbons. Recent findings suggest that they are also involved in the regulation activity-dependent communication between presynaptic boutons and the neuronal nucleus. Together these observations suggest that Bassoon and Piccolo use their modular structure to organize super-molecular complexes essential for various aspects of presynaptic function.

Environmental enrichment: evidence for an unexpected therapeutic influence

Environmental enrichment produces wide-ranging effects in the brain at molecular, cellular, network, and behavioral levels. The changes in neuronal plasticity are driven by changes in neurotransmitters, neurotrophic factors, neuronal morphology, neurogenesis, network properties of the brain, and behavioral correlates of learning and memory. Exposure to an enriched environment has also demonstrated intriguing possibilities for treatment of a variety of neurodegenerative diseases including Huntington's disease, Alzheimer's disease, and Parkinson's disease. The effect of environmental enrichment in epilepsy, a neurodegenerative disorder with pathological neuronal plasticity, is of considerable interest. Recent reports of the effect of environmental enrichment in the Bassoon mutant mouse, a genetic model of early onset epilepsy, provides a significant addition to the literature in this area.

Active zone protein expression changes at the key stages of cerebellar cortex neurogenesis in the rat

Signal transduction and neurotransmitter release in the vertebrate central nervous system are confined to the structurally complex presynaptic electron dense projections called "active zones." Although the nature of these projections remains a mystery, genetic and biochemical work has provided evidence for the active zone (AZ) associated proteins i.e. Piccolo/Aczonin, Bassoon, RIM1/Unc10, Munc13/Unc13, Liprin-α/SYD2/Dliprin and ELKS/CAST/BRP and their specific molecular functions. It still remains unclear, however, what their precise contribution is to the AZ assembly. In our project, we studied in Wistar rats the temporal and spatial distribution of AZ proteins and their colocalization with Synaptophysin in the developing cerebellar cortex at key stages of cerebellum neurogenesis. Our study demonstrated that AZ proteins were already present at the very early stages of cerebellar neurogenesis and exhibited distinct spatial and temporal variations in immunoexpression throughout the course of the study. Colocalization analysis revealed that the colocalization pattern was time-dependent and different for each studied protein. The highest collective mean percentage of colocalization (>85%) was observed at postnatal day (PD) 5, followed by PD10 (>83%) and PD15 (>80%). The findings of our study shed light on AZ protein immunoexpression changes during cerebellar cortex neurogenesis and help frame a hypothetical model of AZ assembly.